The present invention relates to buoyancy “cans” used to provide uplift force to top-tensional risers.
Vast oil reservoirs have recently been discovered in very deep waters around the world, principally in the Gulf of Mexico, Brazil and West Africa. Water depths for these discoveries range from 1500 to nearly 10,000 ft. Conventional offshore oil production methods using a fixed, truss-type platform are not suitable for these water depths, where these platforms become dynamically active (flexible). Stiffening them to avoid excessive and damaging dynamic responses to wave forces is prohibitively expensive.
Deep water oil and gas production has thus turned to new technologies based on floating production systems. These systems come in several forms, but all of them rely on buoyancy for support and some form of a mooring system for lateral restraint against the environmental forces of wind, waves and current.
These floating production systems (FPS) sometimes are used for drilling as well as production. They are also sometimes used for storing oil for offloading to a tanker. This is most common in Brazil and West Africa, but not in Gulf of Mexico as of yet. In the Gulf of Mexico, oil and gas are exported through pipelines to shore.
Drilling, production and export all require some form of vertical conduit through the water column between the sea floor and the FPS. These conduits are usually in the form of pipes which are called “risers.” Typical risers are either vertical (or nearly vertical) pipes held up at the surface by tensioning devices; supported at the top and formed in a modified catenary shape to the sea bed; or steel pipe which is also supported at the top and configured in a catenary to the sea bed (Steel Catenary Risers—commonly known as SCRs).
The flexible and SCR type risers are, in most cases, directly attached to the floating vessel. Their catenary shapes allow them to comply with the motions of the FPS due to environmental forces. These motions can be as much as 10-20% of the water depth horizontally, and 10s of ft vertically, depending on the type of vessel, mooring and location.
Top-tensioned risers (TTRs) typically need to have higher tensions than the flexible risers, and the vertical motions of the vessel need to be isolated from the risers. TTRs have significant advantages for production over the other forms of risers, however, because they allow the wells to be drilled directly from the FPS, avoiding an expensive separate floating drilling rig.
TTR tensioning systems are a technical challenge, especially in very deep water where the required top tensions can be 1000 tons or more. Some types of FPS vessels, e.g. ship-shaped hulls, have extreme motions which are too large for TTRs. These types of vessels are only suitable for flexible risers. Other, low-heave (vertical motion) FPS designs are suitable for TTRs. This includes tension-leg platforms (TLPs), semi-submersibles and SPARs, all of which are in service today.
Of these, only the TLP and SPAR platforms use TTR production risers. Semi-submersibles use TTRs for drilling risers, but these must be disconnected in extreme weather. Production risers need to be designed to remain connected to the seabed in extreme events, typically the 100 year return period storm. Only very stable vessels are suitable for this.
SPAR-type platforms recently used in the Gulf of Mexico use a passive means for tensioning the risers. These types of platforms have a very deep draft with a centerwell, through which the risers pass. Buoyancy cans inside the centerwell provide the top tension for the risers. See, e.g., U.S. Pat. Nos. 5,873,416, 5,881,815, and 5,706,897, all of which are incorporated herein by reference.
Buoyancy cans are typically cylindrical, and they are separated from each other by a rectangular guide structure. These guides are attached to the hull. As the hull moves, the risers are deflected horizontally with the guides. However, the risers are tied to the seafloor; hence, as the vessel heaves, the guides slide up and down relative to the buoyancy can and risers (from the viewpoint of a person on the vessel it appears as if the risers are sliding in the guides).
Referring now to FIG. 1, a typical top-tensioned riser is seen. A wellhead at the sea floor connects the well casing (below the sea floor) to the riser with a tieback connector. The riser, typically a 9-14″ pipe, passes from the tieback connector through the bottom of the SPAR and into the centerwell. Inside the centerwell the riser passes through a stem pipe, or conduit, which goes through the center of the buoyancy cans. This stem extends above the buoyancy cans themselves and are connected to the surface tree. The buoyancy cans need to provide enough buoyancy to support the required top tension in the risers, the weight of the cans and stem, and the weight of the surface wellhead. Since the surface wellhead (“dry tree”) move up and down, relative to the vessel, flexible jumper lines connect the wellhead to a manifold which carries the product to a processing facility to separate water, oil and gas from the well stream.
The underlying principal of buoyancy cans is to remove a load-bearing connection between the floating vessel and the risers. As production and drilling developments go deeper, the connection problem between risers and the floating structure becomes more complex. Buoyancy cans eliminate the need for a load-bearing connection between the two; the cans hold the weight of the riser. The risers are connected to the vessel by flexible pipes that do not hold the riser.
Buoyancy cans are designed to accommodate the weight they need to support and the environmental conditions they are expected to encounter (including specific static and dynamic forces that act on the cans due to the relative motion between the vessel and the cans). Typical buoyancy can designs use steel to resist side-loads due to dynamic motion between the riser and the vessel. As depth increases, the size of conventional buoyancy cans increases along with the thickness of the buoyancy can wall to resist increased pressure at depth. These conditions lead to an increase in thickness of the wall of the buoyancy can, and thus an increase in the weight and cost of the buoyancy can. Furthermore, as the buoyancy can moves within a vessel riser bay, the buoyancy can surface and the guide move against each other in a constant sliding action.
Typical buoyancy cans comprise a large steel sheet rolled to form a pipe around the stem of the riser arrangement. End caps, as well as horizontal bulk heads, are used to transfer the uplift force to the riser arrangement It is difficult and expensive to manufacture buoyancy cans with such a configuration. Thus, there is a need for a simpler design for buoyancy cans, simpler methods of manufacturing buoyancy cans, and there is a need for a lighter buoyancy can. Furthermore, there is a need for a buoyancy can that is cheaper to build, smaller in diameter and length, and easier to fabricate and install.